This invention relates to optical elements for wavelength selection and more particularly to air spaced etalons having a high resolving power.
As is well known, an etalon is often used with an ion laser, for example, to achieve single frequency operation. This optical element is in effect a sort of Fabry-Perot resonator having a resonator distance l much smaller than that, L, of the laser resonator, and, for use with a laser, is accommodated in the laser resonator. Generally, in an ion laser, the Doppler-broadened gain width of the laser medium due to the Doppler effect amounts to several GigaHertz and oscillation may occur over such frequency width. Actually, however, because of the very high Q-value of the laser resonator, there exist several tens of spectrum lines with a longitudinal mode spacing determined by c/2L, where c represents the velocity of light and L represents the distance of the reflectors forming the resonator.
Such laser output makes the laser undesirable as a light source for use in holography and other similar information processing techniques utilizing light interference or in fields of spectroscopy where the scattered light dealt with is of limited frequency deviation as in Brillouin scattering. Namely, in the case of laser application to holography or the like, interference information produced by one longitudinal mode spectrum may possibly be cancelled out by another longitudinal mode spectrum and, in laser applications in the field of spectroscopy, it makes it impossible to separate frequency deviation of the Brillouin scattered light from the Rayleigh scattered light. To cope with these situations, however, it is possible to realize a single-frequency oscillation ion laser by inserting an etalon in the resonator of the ion laser oscillating at different frequencies in effect to form a sort of composite resonator. In the laser formed in this manner, oscillation takes place at only one of the several tens of spectrum lines within the Doppler width, that is, at that spectrum line which is closest to the resonance frequency of the etalon.
Use of an etalon with an ion laser has been illustrated above, but it also has many other uses. For example, in a single-frequency oscillation ruby laser, which is now employed in a variety of schemes for obtaining measurements of moving objects, an etalon is being employed for single frequency operation against the natural frequency bandwidth of the laser medium, which is said to amount to approximately 300 GHz. Further, an etalon is now indispensable as a convenient optical element usable to narrow the oscillation width of a dye laser, which is drawing attention in the field of spectroscopy.
Etalons previously in use, however, have generally taken the form of a quartz plate having a pair of opposite surfaces polished so as to be parallel to each other with a reflection coating of a reflectivity of about 20% deposited on each of the polished surfaces. As is well known, quartz is a good optical material of limited transmission loss and, due to its limited coefficient of thermal expansion, the resonance frequency of the previous form of etalon, which is determined by the thickness of the quartz plate, has only a limited temperature change of the order of .+-.125 MHz/.degree. C. Recently, however, more stable etalons are being desired for use with ion lasers. Under this situation, crystalline glass materials of still lower thermal expansion coefficients such as "Neoceram-Zero" (trademark) made by Nippon Electric Glass K.K., Japan, and "Cervit" (trademark) made by B.T.R. Optics, Inc., U.S.A., are now commercially available and are being employed to form etalons which exhibit better stability under temperature change.
Actually, however, none of such crystalline glass materials can be used in the same formation as the quartz plate described hereinbefore because of the larger optical loss involved and they are therefore used to form a hollow cylindrical spacer, the opposite end surfaces of which are polished parallel to each other. Partially reflective mirrors are bonded to the respective polished end surfaces of the spacer to complete an air spaced etalon. With such form of air space etalon employing as a spacer material "Neoceram-Zero" glass, the coefficient of thermal expansion of which is substantially zero, the temperature variation of the resonance frequency has been found reduced to .+-.17 MHz/.degree. C. or to about one-seventh of that of the conventional form of etalon employing a quartz plate.
Use of such a hollow cylindrical spacer, however, has previously involved some disadvantages. First, in the step of polishing the opposite end surfaces of such spacer, it can hardly be held in balance as there is no direct axial force application along the axis of the spacer because of its hollow formation as contrasted to the solid formation of quartz plates. It is very difficult, therefore, to obtain an air spaced etalon having the required parallelism of two seconds or less and this has obviously resulted in a poor yield of polishing operation and a high cost of production.